This is a real Firefox bug, but consider the reverse situation: if Firefox's slice() was fast and Chrome's was slow, would Google ship Inbox with a message that says "Inbox only works in Mozilla Firefox. More browsers coming soon. Download Mozilla Firefox."? Probably not. The Inbox team would probably pull in the V8 team. Apparently, the Inbox team didn't contact Mozilla's SpiderMonkey team because they fixed their bug within a couple hours:
https://bugzilla.mozilla.org/show_bug.cgi?id=1087963